1. Field of the Invention
The present invention relates to a generation method of a light intensity distribution, a generation apparatus of a light intensity distribution and a light modulation element assembly; for example, the present invention relates to measurement of a light intensity distribution of light applied to a predetermined flat surface or plane in a light intensity distribution generation apparatus used in a crystallization apparatus.
2. Description of the Related Art
In recent years, there has been developed a technology which uses a laser beam to perform a wide variety of processing such as free-form curve cutting, boring, welding, surface finishing, microfabrication or the like with respect to various kinds of materials such as iron, non-ferrous metals, ceramics, plastics, wood, fabrics, paper, and composite materials. For example, surface fishing processing is processing which irradiates a surface of a material with a laser beam having a relatively low energy density to heat a surface part only, thereby reforming the heated part. In this surface finishing processing, there are cases where the surface part is processed in a solid state and where the surface part is fused to be processed.
In a display device such as an active matrix type liquid crystal display device or an organic electroluminescent display device, many thin film transistors [TFTs] are formed on an insulating substrate such as glass or plastic in order to individually drive pixels for display. An amorphous silicon (a-Si) film in which source, drain and channel regions of a TFT has a low formation temperature, can be relatively easily formed by a gas phase method and is superior in mass productivity. Therefore, this film is generally used as a semiconductor film utilized for the TFT.
Such an amorphous silicon film has a drawback that its physical properties, e.g., electrical conductivity, are inferior to those of a poly-silicon (p-Si) film (the mobility of a-Si is two or more digits lower than that of p-Si). Therefore, in order to increase an operating speed of the TFT, there is used a technology which forms an a-Si film, changes this a-Si film into a p-Si film and forms source, drain and channel regions of the TFT in this polycrystal silicon film, e.g., an annealing method (Excimer Laser Annealing; which will be referred to as an “ELA method” hereinafter) using an excimer laser. Since this ELA method can be carried out in a temperature range where a general-purpose glass substrate can be used, i.e., a temperature range from a room temperature to approximately 500° C., it has the advantage that the material of the substrate is not restricted.
The ELA method is, e.g., a crystallization method which deposits an a-Si film on a substrate to provide a predetermined thickness (e.g., a thickness of approximately 50 nm), and then irradiates this a-Si film with a laser beam such as a krypton fluorine (KrF) excimer laser beam having a wavelength of 248 nm or a xenon chlorine (XeCl) excimer laser beam having a wavelength of 308 nm to locally fuse/recrystallize the a-Si film in an irradiated region, whereby the a-Si film is changed into a p-Si film.
The ELA method can be adapted to any other various processes by appropriately selecting an average intensity (a fluence) of a laser beam. For example, when a laser beam is set to an intensity with which a heating function alone is demonstrated, the ELA method can be used for an impurity activation step of a TFT. Further, when the intensity of a laser beam is set to be extremely large, a sudden increase in temperature is provoked, and hence the ELA method can be also utilized for removal of a film in the TFT. Furthermore, utilization of these phenomena is not restricted to the TFT and can be extensively adapted to a semiconductor manufacturing process.
In a display device such as a liquid crystal display device or an organic electroluminescent display device, when a TFT is formed in a p-Si film in order to increase operating speed, crystal grain boundaries of the p-Si film exist in a channel region of the TFT. In this case, the number of the crystal grain boundaries formed in the channel region differs in accordance with each TFT, and hence this difference in the number of the crystal grain boundaries considerably increases irregularities in characteristics such as threshold voltage or a mobility of each TFT. Such irregularities in threshold value in each TFT greatly lower operating characteristics of the entire display device, which can be a factor deteriorating a picture quality or the like.
Therefore, there has been a demand for equalizing the number of crystal grain boundaries in a channel region of each TFT as much as possible or eliminating crystal grain boundaries from the channel region of each TFT thus it is desired to form a crystallized region having a large particle or grain diameter and controlling a crystallized region forming position so that the TFT can be formed in the crystallized region. The present inventors have carried out development with respect to such a demand, whereby a crystallized region having a large particle diameter can be manufactured by using a light modulation element (see Jpn. Pat. Appln. KOKAI Publication No. 2004-186449 and Jpn. Pat. Appln. KOKAI Publication No. 2004-193229).
Jpn. Pat. Appln. KOKAI Publication No. 2004-186449 reveals that, in development of industrialization of this technology, evaluation and management of a light intensity distribution of a laser beam which is applied to an a-Si film as a crystallization processing target body in an accuracy of submicron order are very important in an increase in grain diameter and positional control of a crystallized region. Especially, in a mass production line, periodical monitoring of a light intensity distribution is important in order to use a laser beam source which relatively has a problem in output stability. However, since the light intensity distribution has a fine structure of a submicron level and an excimer laser beam preferable for crystallization is invisible, there is a problem that monitoring with the naked eye of an operator is difficult.
Therefore, the present inventors have carried out development for visualization of a light intensity distribution to obtain an apparatus having a configuration in which an original light modulation element and another light modulation element called a visualization mask are provided in a laser beam path of a crystallization apparatus. The apparatus irradiates an a-Si film with a laser beam to perform crystallization with a large particle diameter. The a-Si film is irradiated with a light-modulated laser beam through these two light modulation elements, thereby realizing visualization. This visualization apparatus fuses an irradiation target surface of an a-Si film by application of a laser beam. A fused region is crystallized in a temperature reducing process when the laser beam is interrupted. Physical properties of the thus formed crystallized region are changed. A method of utilizing this change in physical properties to visualize a light intensity distribution has been developed. The present inventors have released an optical system as means for accurately measuring a light intensity distribution in an internal academic conference (IDW'04, Proceedings of the Eleventh International Display Workshops).
In general, as a method of generating a predetermined light intensity distribution, there are methods of aligning an element pattern which modulates an optical amplitude, an element pattern which modulates an optical phase, and an element pattern which modulates both an optical amplitude and an optical phase while changing a modulation quantity. Here, reducing a size of the element pattern to be smaller than a point spread range of an image forming optical system can remove a shape of the element pattern from a light intensity distribution to be generated, thereby realizing a smooth distribution. That is, just performing binary processing of the light modulation element can generate a light intensity distribution having a predetermined gradation.
Here, as shown in FIGS. 19A and 19B, a consideration will now be given on a light intensity distribution which is generated when two light modulation elements (a first light modulation element [FIG. 19A] and a second light modulation element [FIG. 19B]) having one-dimensional patterns orthogonal to each other are superimposed without a gap therebetween. Even if a relative position of the two light modulation elements is shifted, since an overlap pattern itself is not changed as shown in FIG. 19C, a light intensity distribution to be generated is laterally shifted as a whole but not changed. On the other hand, as shown in FIGS. 20A and 20B, where both light modulation elements have two-dimensional patterns or at least one of the two light modulation elements has a two-dimensional pattern although not shown, shifting of a relative position of the two light modulation elements changes an overlap pattern as shown in FIGS. 20C and 20D and also varies a light intensity distribution to be generated.
For example, it is assumed that the first and the second light modulation elements shown in FIGS. 20A and 20B are an optical amplitude type light modulation element and a hatched part in the figures indicates a light shielding region whilst a blank part indicates a transmission region. In this case, the transmission region does not exist in an overlap pattern in a FIG. 20C state where the two light modulation elements are superimposed. In the other hand, relatively large openings (the transmission regions) are generated in the overlap pattern by half-pitch shifting of an element pattern in a FIG. 20D state where the two light modulation elements are superimposed. In these two different superimposed states, it can be expected that light intensity distributions to be generated are also greatly different from each other.
Moreover, when a relative angle (a rotating angle around an axial line vertical to a page space of FIGS. 20A and 20B) of the first and second light modulation elements is changed, the overlap pattern differs depending on each position. Thus results in a change, i.e., irregularities in the light intensity distribution to be generated depending on each position. Additionally, if a pitch of the element pattern is slightly different between the first and second light modulation elements, irregularities with a large cycle are generated in a light intensity distribution to be generated. Further, even if each of the first and second light modulation elements is an optical phase type light modulation element, and even if one of the light modulation elements is an optical amplitude type light modulation element and the other one is an optical phase type light modulation element, a change or irregularities are likewise generated in the light intensity distribution. It has been revealed that such a change or irregularities in light intensity distribution are apt to be produced when an area modulation type phase shifter which can be readily designed as a light modulation pattern is used.
Furthermore, although the element patterns are the same in a plane in FIGS. 20A and 20B, when the element patterns vary in a plane, it can be understood from considerations that the light intensity distribution to be generated is changed in accordance with relative displacement or the like of the two light modulation elements. In this specification, a phenomenon that irregularities or a change is generated in a light intensity distribution to be produced in accordance with relative displacement or the like of the two light modulation elements in this manner is called the “Moire effect”. In the measuring method and the measuring apparatus of a light intensity distribution proposed in the above-described patent application, a desired two-dimensional light intensity distribution cannot be generated due to an influence of the Moire effect caused by relative displacement or the like of two light modulation elements, and a light intensity distribution formed by a measurement target light modulation element alone cannot be accurately measured.